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When I bought the tank it came with the fish, rocks, plants etc. The fish were 4x neon tetra, 4x harlequins, 4x glow-light tetras and a chinese algae eater.

First off, how did you bring the tank and fish home? I'm assuming the fish were put in a bucket of tank water and the tank emptied for transport? In this case, what did you do with the filter and tank substrate? The filter is what holds your biomedia, whether with ceramic beads, filter sponge ect. The substrate is also very important for this reason. Therefor, if EITHER became dry or dried out, your biomedia could've died. When that happens the tank must be fully re-cycled to restore it to it's previous state as creating live biomedia is the point of cycling.

Originally Posted by jwal90

I wouldn't say there's an issue with the size, I picked out the smallest angel fish I could find and he's only around 3 times bigger than the tetra.

This shows a lack of research to your chosen fish species. Angelfish get quite large, they can get to be anywhere from 6-8" NOT including finnage.

Originally Posted by jwal90

I did changed one of the filter pads but it doesn't seem to have caused any issues. I also added some 'general tonic' which came with the tank when I bought it because it was supposed to help with diseases etc.

Again, please refer to my above mention of the filter holding your biomedia. Ideally you do NOT want to change out your filter pad, instead, during a water change, save a bucket of tank water to rinse out your filter sponge or pad depending on your model. Changing the filter pad out REMOVES the live biomedia and therefore, takes your tank back to step one for cycling as if a fresh tank. It is okay to completely replace charcoal however.

All in all, I believe your tank, though it was previously established, is back to being a new tank that has yet to cycle. DO NOT buy anymore fish and instead IMMEDIATELY get your hands on a water testing kit, liquid drops are more reliable than the strips. Monitor your tank until reach the cycled parameters of ammonia: 0, nitrite: 0, nitrate: 5-15. Those are the basic parameters of a cycled tank.

The filter has 2 pads so I replaced one of them with a new one I had soaked in a sample of the tank water.

The dimensions of the tank are as follows: w: 58cm d: 34cm h: 39cm (height measurement representing water level not tank level)

The fish were transported in the tank by moving half of the water into a bucket and removing rocks etc. The filter was taken from the tank but was replaced within 10 minutes as I only lived round the corner from the seller.

The angel actually seems to have perked up and spent a majority of the time in the centre of the tank, however, the bamboo shrimp seems to have gone from very active to rather subdued in the last 2 days.

Water temp levels have been between 23 and 27 degrees celcius since we have the tank but now seem to be sitting around the 24 mark. This was due to a problem with my stat.

actually, it sounds as though you were able to transfer the tank without losing your cycle. Perhaps the angel was simply a bit out of sorts do being moved to a new home. I'm glad things are looking up for you.
You need to shoot for 0 ammonia as well as 0 nitrites. the .25 ammonia reading could have been a mini spike due to the move and changing out the filter pad.

For ongoing maintenance, try to make a 30 - 50% water change weekly. Also, as was mentioned, no need to change out your filter pads (even though the mfg instructions say so) simply rinse in old tank water when your nitrates start elevating and all should be well. I'd also try to get the temp at a steady 25 or 26C

The remaining problem now is the size of the tank for the angel. You're not even looking at 15 gallons and the very smallest tank a single angel should be in is 30 gallon. It may be small now but angels get quite large and that tank will not be big enough to accommodate it. If you like the angel and want to keep it, please have a plan in mind for down the road. like a larger tank :o) If you don't see a larger tank happening, I'd return the angel then leave the tank stocked as it is with no more fish because there simply isn't room.
much luck and I hope this has helped you and your fish

That's great news, I'll be doing a 50% water change this weekend and will be keeping a regular eye on the levels. Really appreciate the help here, I probably did jump the gun without doing my research but glad I've learned a few things and the fish are ok.

I'll keep an eye on the angel and if he get's too big I will have to take him back from the garden centre I purchased him from. Although I am a little put out as the staff there assured me he would be fine in my current tank with the other fish.

I do have another question though. The bamboo shrimp and algae eater have both swam into a small gap between the back of the tank and the filter. They're not stuck or anything but they haven't moved for most of the day. Is this normal behavior?

I'll keep an eye on the angel and if he get's too big I will have to take him back from the garden centre I purchased him from. Although I am a little put out as the staff there assured me he would be fine in my current tank with the other fish.\ I don't mean to harp on this (well, maybe I do) but it's not a question of IF the angel will get too large. If he lives, he will be too large. And placing him in this small tank is not fair to him either way as the small tank could stunt his growth and not in a good way. If you are determined to keep him, you need to buy a larger tank. Minimum 29 gallons - more than double the size you have now. And yes, the fish store should have known better. Again - harping, sorry, but you should return the angel asap as your water quality will rapidly degrade as he grows. your tank is fully stocked without the angel.

I do have another question though. The bamboo shrimp and algae eater have both swam into a small gap between the back of the tank and the filter. They're not stuck or anything but they haven't moved for most of the day. Is this normal behavior?

I don't know anything about shrimp except that they like to hide as does the algae eater. So I don't think it's unusual for that kind of behavior. they are more active at night.

Generally bamboo shrimp set up shop in one spot near flowing water and spend their whole life there filter feeding. Its not always going to be easy to see but its life is not spent moving from spot to spot searching for food but is sedentary and filters little things out of the water. That is a very big reason they are recommended only in well established tanks.

I'll keep an eye on the angel and if he get's too big I will have to take him back from the garden centre I purchased him from. Although I am a little put out as the staff there assured me he would be fine in my current tank with the other fish.

Well.... Garden center staff are world renown for their lack of fish knowledge [But it's your job to know what can go in your tank anyway] & It's not all about the angels size, Please re-read post #9.

Gas mileage isn't everythingOIIIIIIIOLack of planning on your part does not constitute an emergency on my part.Why pretend there are no stupid questions? Actually, There are many stupid questions: "Should I drink this bleach?" Is just one example.
Having said that, Just because it's a stupid question doesn't mean that it shouldn't be asked. It's better to know.A warm beer is better than a cold beer. Because nothing is better than a cold beer, and a warm beer is better than nothing.